This application is a continuation of PCT international application no. PCT/EP01/05641, filed May 17, 2001, and claims priority based on German patent application no. DE 100 26 688.6 filed May 30, 2000.
This application is a continuation of PCT international application no. PCT/EP01/05641, filed May 17, 2001, and claims priority based on German patent application no. DE 100 26 688.6 filed May 30, 2000, the specifications of which are incorporated by reference herein.
The present invention relates to a method for determining the response pressure of vehicle brakes particularly a road vehicle, by a) admitting a test pressure to the brakes to be examined, with and without test pressure, b) measuring rotational wheel speed signals at at least two axles of the vehicle, c) from measured rotational wheel speed signals, determining an acceleration signal, and d) after the application of the test pressure, determining the brake pressure at the brake or brakes to be examined.
A device for determining the response pressure of the brakes of a vehicle is also provided.
A method of the above-mentioned type is known from European Patent Document EP 0 733 532 A2.
In the case of a vehicle brake system, a certain minimal brake pressure must be exceeded to generate a braking force. This minimal brake pressure is called the xe2x80x9cresponse pressurexe2x80x9d. A vehicle braking system normally has several brakes, which may have different response pressures. This results in an undesirably high wear on the first-responding brakes. The wear problems are particularly significant when a towing vehicle is coupled with different trailers because the amount of the response pressure of individual vehicles depends on the individual restoring forces and the mechanical friction between individual brake components. The determination of the response pressure may take place, for example, on a roller-type test stand which, however, is relatively cumbersome.
From the above-mentioned European Patent Document EP 0 733 532 A2, a method is known in which the response pressure is determined during the drive, specifically by analyzing the vehicle acceleration. From rotational wheel speed signals, first a possibly existing vehicle deceleration is determined, which is called a xe2x80x9cbasic vehicle decelerationxe2x80x9d and is caused, for example, by wind resistance, frictional forces or a slope inclination. Subsequently, the brake to be checked or the group of brakes to be checked are acted upon by a test pressure and the test vehicle deceleration which will then occur is determined. When the test vehicle deceleration does not change with respect to the basic vehicle deceleration, the test pressure is increased. If the vehicle deceleration increases and the difference between the test vehicle deceleration and the basic vehicle deceleration is greater than a defined tolerance value, the existing brake pressure is considered to be the response pressure of the brake. When analyzing rotational wheel speed signals, in practice, only relatively rough xe2x80x9cdeceleration stagesxe2x80x9d of approximately 0.2 m/s2 can be detected. Consequently, in this state of the art, a certain xe2x80x9cminimum test pressurexe2x80x9d is required in order to reach a detectable deceleration change, which is perceived by the driver as a disturbing xe2x80x9cjerkingxe2x80x9d. Also, a vehicle deceleration as a result of a change of the engine power cannot be differentiated from a vehicle deceleration as a result of the testing brake pressure.
From German Patent Document DE 196 02 879 C1, a method is known for detecting the turnover risk of a vehicle, in the case of which the lateral vehicle acceleration is constantly monitored and, when a defined limit value is exceeded a test braking is carried out at a low braking force. It is then examined whether the ABS (antilocking system) is responding at individual wheels or wheel groups and is not responding at other wheels.
German Patent Document DE 196 48 936 A1 describes a method which determines the application pressures by means of a statistical analysis of measuring dataxe2x80x94using a regression analysis. The measurements take place during service brakings, which therefore results in long identification times.
It is an object of the present invention to provide an improved method and an improved device for determining the response pressure of vehicle brakes.
This object is achieved by a method and device for determining the brake response pressure, including the steps of:
a) admitting a test pressure to the brakes to be examined, with and without test pressure,
b) measuring rotational wheel speed signals at at least two axles of the vehicle;
c) from measured rotational wheel speed signals, determining an acceleration signal,
d) after the application of the test pressure, determining the brake pressure at the brake or brakes to be examined;
e) in each case, for a driving condition with and without applying the test pressure, forming a differential speed signal (s) as a function of a difference between rotational wheel speed signals of a powered and of a non-powered axle;
f) forming test pressure indicator signals, respectively, each as a function of the acceleration signal and the differential speed signal; and
g) comparing two test pressure indicator signals formed, and repeating steps (a)-(f) with an increased test pressure when the two test pressure indicator signals coincide, and emitting the applied test pressure when the two test pressure indicator signals do not coincide.
Advantageous developments and further developments are described herein.
The invention is based on the idea of admitting a test pressure to the brakes to be examined, in which case, before and after generating the test pressure, a test pressure indicating signal is generated. This signal is a linear combination of an acceleration signal and of a differential speed signal. If the two test pressure indicator signals correspond to one another within a tolerance band, which is considered to be a xe2x80x9cnon-response of the brakexe2x80x9d, the test pressure is increased and a new test pressure indicator signal is generated. If the two test pressure indicator signals do not correspond, the existing brake pressure is considered to be the response pressure if the difference between the two test pressure indictor signals exceeds a defined limit value.
While, in the state of the art, the response of the brake is recognized only by the analysis of the vehicle deceleration, the test pressure indicator signal used in the case of the invention, in addition to the acceleration signal, also includes a differential speed signal, which can also be called a xe2x80x9cslip signalxe2x80x9d and is formed of rotational wheel speed signals of an axle acted upon by the test pressure (or not acted upon by test pressure) or a wheel and a powered or non-powered vehicle axle.
Because the test pressure indicator signal according to the invention includes the acceleration signal and the differential speed signal, a response of the brake can already be recognized at lower test pressures. Another advantage of the use of the acceleration signal and of the differential speed signal or slip signal is the fact that a change of deceleration caused by the test pressure can be differentiated from a deceleration change, which is a result of a change in engine power, for example, as a result of applying gas or shifting-down into a lower gear. The change of the vehicle deceleration (jerking) caused by the test pressure is also reduced.
According to a further development of the invention, it is provided that, when the test pressure is generated, the vehicle deceleration is kept approximately constant by increasing the engine torque, which further increases the driving comfort. Furthermore, for example, a xe2x80x9ccombinationxe2x80x9d of a device according to the invention with a conventional cruise control may be provided so that the vehicle speed is kept constant. Also in this case, the test pressure indicator signal becomes sufficiently large since its slip component will grow because of the increased engine torque.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.